Monday, October 23, 2017

Update

A clue
regarding the cipher system used is available from the TICOM report DF-241
‘The Forschungsamt - Part IV’, p40

‘Of the numerous examples which might be
adduced, the following may serve as an example: The additive number used by Great
Britain, which ran to 40,000 elements and served for the encipherment of the
5-digit code and was replaced at definite intervals of time, offered as a rule
adequate assurance of security. But if in periods of greatly increased
diplomatic activity with telegraphic traffic many times the usual volume the additive
is not replaced correspondingly sooner, especially since increased security is
desirable in such periods, then this is a sign of deficient control’.

Thus it is
possible that the German codebreakers were able to solve the British Foreign
Office cipher in the 1930’s.

The official
history ‘British Intelligence in the Second World War’ - vol2, p642 says that:

‘FOREIGN OFFICE

1. Main Cypher Books

Despite an extensive attack in 1938
and 1939, the Germans failed to break the long subtractor system used to
re-cypher the Foreign Office's basic cypher books. Against similar tables that
were in force from November 1940 to January 1941 they had some limited success,
but not enough to enable them to reconstruct the book before both the basic
book and the tables were again changed. There is no evidence of later success,
and according to German testimony after the war the main Foreign Office systems
were never broken’.

However in
the notes it also says:

‘The discovery after the war in the archives
of the German Ministry of Foreign Affairs of
a 90-page volume of British diplomatic signals for the immediately
pre-war period led to a Foreign Office
enquiry in 1968. This established that a number of the signals had been
dispatched en clair. It also noted that there was reliable evidence that the
Italians had obtained temporary possession of the cyphers of the Rome Embassy
in 1935, and had photographed them, and that they had had fairly regular access
to the cyphers at the Mission to the Holy See during the war, so that they
might have read all telegrams to Rome up to the outbreak of war and telegrams
to and from the Mission to the Holy See from the outbreak of war to the autumn
of 1943. After the war the cryptanalysts of the German Foreign Ministry
asserted that they obtained no information about British cyphers from the
Italians’.

The British
statements may have been accurate about the work of the decryption department
of the German Foreign Ministry but they do not mention the Forschungsamt
effort…